Differential boiler



Jan. 10, 1950 H. o. MOMAHON 2,494,304

DIFFERENTIAL BOILER Filed June 5, 1946 w. wf 3 WNW M67530 7 .fifajgaz d 011671422070 i utented .ian. 10, 1950 'DIFFERENTIAL noman Howard 0. McMahon, Le ington, Mass assignor, by mesne assignments, to Arthur D. Little, Inc" Cambridge, Mass., a corporation of Massachusetts Application June 5, 1946, Serial No. 674,521

9 i 1 Claim. ((162-122) This invention relates to a i'ractionating boiler for separating condensed or liquefied fluids of diiferent boiling points and is illustrative of its utility it is herein shown for use in connection with an automatically controlled oxygen-producing apparatus of the type disclosed in co-pending application Serial No. 661,253 filed April 11, 1946. by Samuel C. Collins.

The principal object of the invention is to provide a reliable and eflicient method of and apparatus for effecting at least a partial separation and segregation of the components oi. a condensed fluid mixture. More specific objects are to provide an efficient and reliable method'and apparatus whereby substantially pure oxygen, either in liquid or gaseous phase, may be obtained from liquefied air. Further objects will be apparent from a consideration of the following description.

In accordance with the present invention a condensed or liquefied fluid is brought into heat exchange relation with a liqueflable gaseous fluid (containing an appreciable amount of heat) in such a manner as to establish and maintain a counter-current flow between the two fluids whereby the liquefied fluid absorbs from the gaseous fluid suflicient heat to effect a progressive evaporation of at least a part of its components and the gaseous fluid is cooled-correspondingly. To this end I provide an elongate enclosed channel which is disposed in heat exchange relation 2 the fluid travels toward the opposite or exit end of the channel. Where the channel containing liquefled fluid is 01' substantially uniform crosssectional area, the volume or the vaporized fraction progressively increases and hence may be forced to flow back towards the point of admisskin where it is permitted to escape through a suitable conduit, thus becoming segregated from the body of liquefied fluid moving toward the exit end of the channel. The longer the channel the greater the enriching action, but in any event to a second channel, each channel having means for admitting fluid at one end and means for withdrawing a gaseous and/or liquefied fluid at a point remote from the point of admission of fluid thereto.

:A liquefied mixture of two or more components having substantially different boiling points, such asa mixture of liquefied gaseous components normally present in air or other multicomponent mixture, is admitted to one of the channels at a predetermined rate so as to flow substantially horizontally through the channel at a level below that of the top of the channel to provide a space ofappreciable depth above the liquid, and simultaneously there is admitted to the other channel a liquefiaolc gaseous fluid, such as compressed air at a relatively low temperature, so as to flow in a direction counter to that of the fluid in the first channel. Due to the heat exchange between the gaseous and liquefied fluids as each travels from one end toward the other end 01' its channel. progressively increasing ebullition of the liquefied fluid takes place with the result that the more volatile constituents aredriven of! in decreasing amounts as the fluid travels from its point of admission toward the opposite end of the channel. Hence, both the liquid and gaseous phases become more concentrated or enriched in the higher boiling point constituent as now considered under lire-established operating conditions an enriched liquefied fluid and/or gas of prede-- termined purity may be withdrawn at a point remote from the point of admission.

Simultaneously the gaseous fluid. admitted to the other channel in traveling from its point of admission continually gives up heat to the liquefied fluid and hence undergoes a decrease in volume. Depending upon the length of the channel and the particular operating conditions, the gaseous fluid may be cooled to or beyond its condensation point before being drawn oil, but in any event, the cooled or condensed fluid is drawn oil at such a rate as to maintain the necessary flow through the channel to insure the desired heat exchange between the gaseous and liquefied fluids.

Where, for example, it is desired to separate the lower boiling or more volatile constituents of a mixture from the high boiling point constituent, the cooled gaseous fluid or condensate may be drawn ofi, passed through a condenser associated with the boiler, and the condensate then fed into the first channel for further fractionating in the manner above described, thus producing a continuous process for separating the components of a fluid mixture which may either be normally liquid or gaseous, provided that the components have appreciably difierent boiling points. I

A specific applicationbf the toregoingis illustrated in the accompanying drawings which show a fractionating boiler designed for use in the low pressure oxygen-producing apparatus disclosed in copending application Serial No. 661,253, filed April 11, 1946.

In the drawings:

Fig. 1 is an elevation of a fractionating boiler associated with a condensing column, heat exchanger, etc., collectively constituting what is a preferred system for producing pure oxygen from air under pressure at areduced temperature; I

Fig. 2 is an enlarged vertical section through the fractionating boiler; and

Fig. 3 is a horizontal section on the line 3-3 of Hg 2 Referring to'Fig. 1 of the drawings, the embodiment shown therein comprises a fractionating boiler l associated with a rectifying column or the like device 2 which is preferably packed with Bcrl saddles, wire mesh or other suitable material. The upper end of the column is equipped with a spray head. rotary distributor or the equivalent device 8 designed uniformly to distribute liquid on the upper end of the packing. The discharge manifold of the boiler I, hereinafter more fully described, is connected with a line I which after passing through a heat elchanger I is connected with a coupling lot the like device having a restricted orifice. The coupling I is connected with a line 1 leading to the distributor 3 and a line 1 having either a thermostatically-operated or a pressure-operated valve is also connected to the distributor 8. The upper end of the column 2 is connected by an outlet line 8 to the heat exchanger I, the lower end of which is provided with an outlet line I connected in the system as shown in the aforesaid conending application Serial No. 661,253, filed April 11, 1946.

Referring to Figs. 2 and 3, the boiler l comprises a cylindrical shell or casing ll, preferably of stainless steel. copper or the like metal, having a bottom wall [I formed with a central opening l2 and a small marginal opening it. The upper end of the boiler is provided with an annular member it by means of which the boiler is directly connected with the flanged lower end it of the column 2 so as to receive condensate therefrom. The upper and lower side wall of the boiler is formed with vertically aligned openings to receive the inwardly-directed ends of a level or overflow tube 2|] having a horizontally-extending side arm 2| positioned to maintain a predetermined liquid level within the boiler, the side arm 2| being associated with a thermostatic control or the like device constituting a part of the aforementioned system.

An elongate rectangular sheet of metal 28, such as a sheet of stainless steel, is coiled up into a spiral and inserted into the shell I. to form a long narrow spiral channel having an inlet or admission end 28 anda discharge end 21. the intermediate convolutions being designated by the letters a, b, c, d, and e. The sheet 25 thus provides a spiral partition or baiile and its upper and lower horizontal edges are soldered, brazed or otherwise secured to a circular bailie plate 22 and a circular plate 29 resting on the bottom wall ll. so as to provide fluid-tight joints indicated by the numeral 38. The baiile plate 28 is disposed at a level below the upper end of the shell.

' Side arm 2i is located in such a position as to insure complete coverage of the boiler tubes 35, and also so as to prevent accumulation of so much liquid as to rise into and flood the fractionating column 2. Even though liquid may accumulate and cover baiile plate 28, the average level of liquid in channels a-e will be considerably below baiile plate 28 because of the vapor being formed around the boiler tubes II by the violent ebullition.

An air inlet manifold 32 projects upwardly through the opening I2 and within the manifold is an oxygen discharge passage or duct 34 having its upper ends spaced below the baille 28. The

Junction of the manifold and lower plate 2! is soldered or otherwise sealed as is also the Junction of the periphery of the plate 29 and the bottom wall I l thereby preventing leakage of fluid at these parts. The periphery of baiile plate 22 isspacedfromtheinner walloftheshellso condensate dropping thereon is carried to the admission end ll and the outer convolution o of the spiral channel. The upper end of the manifold is contracted about the end of the pipe ll so as to provide a fluid-tight joint which supports the outlet pipe in properly centered position within the manifold 22.

The manifold 22 is connected by a duct 23 with an expansion engine or the like device, as shown in the aforesaid copending application, so as to receive air at a predetermined rate and under a relatively constant temperature and pressure, and the oxygen outlet line It may be directly connected with a suitable receiver and/or one or more heat exchangers, as shown in the aforementioned copending application.

A bank of copper tubes 35 (here shown as 20 in number, each being about 44" in length, 0.125" O. 1).; and a wall thickness of .012") is coiled up to provide a spiral fitting within the spiral channel defined by the partition 25. One end of the bank is connected with the air inlet manifold 32 and its opposite end is connected with a discharge manifold 38 located in the outer convolution a. The lower end of the discharge manifold projects through the opening it at the bottom wall of the shell and is connected with the line 4 leading to the heat exchanger I.

The apparatus herein shown is designed to fulfill two major requirements, viz., (l) substantially all of the air must be condensed because any which passes through as vapor is lost with the waste nitrogen and some of the oxygen'is lost with it, (2) the pressure required to effect complete condensation must be held to a minimum because otherwise the back pressure on the expander would be high and the amount of refrigeration produced would be correspondingly low.

The air discharge into the tube 33 is under a pressure of the order of 75 pounds per square inch and at a temperature of approximately --165 C. Under normal operating conditions it is desired that standard cubic feet of air per minute be condensed within the tubes with a pressure drop of about 5 pounds between the inlet manifold 32 and the outlet manifold 38, although it is to be understood that these figures are merely illustrative of the desired operating conditions in producing pure oxygen from compressed air.

Preliminary to normal operation, liquefied air such as producedin the manner described in the aforementioned copending application is introduced into the column 2 through the line 'I' and nozzle 3 with the result that the condensate dropping on the baiile 28 accumulates within the boiler 1. During this preliminary operation the major part of the nitrogen and argon, together with a relatively small amount of oxygen. are

vaporized when passing through the column 2 and such vapor is permitted to escape through the outlet I into the heat exchanger 5. This preliminary operation of liquefaction cycle is eifective to cool down the apparatus to the desired operating temperature and when the accumulation of condensate in the boiler reaches the level of the tube. 2|, it overflows into a thermostat or the like device which in turn is effective to close the line I' and open the line connected with the manifold 22, thus initiating the normal oxygenproducing operation, as more fully described in the aforesaid copending application.

In normal operation compressed air at a low temperature (approximately -154 C.) is canthat densed in passing from the manifold 22 through the tubes 35, transferring its heat to the condensate in the spiral passage a-e. Since the air in the tubes 35 travels in a direction opposite v that of the fiow in the spiral passage, the condensate at and adjacent to the discharge end 21 receives appreciably. more heat than that at and adjacent to the inlet end 26. Consequently increasing ebullition of the condensate takes place as it slowly travels from the outer convolution a to the discharge point 21, and meanwhile the air passing through the tubes 35 is cooled down correspondingly, the relative rates of flow of the two fluids being such that by the time the air traveling through the tubes 35 reaches the manifold 38 substantially complete condensation has taken place. For example, the condensate from the column 2 which drops into the boiler may consist of about 90% oxygen and 10% nitrogen, argon, etc., and its temperature when passing through the outer convolution may be of the order of -178 C. As the condensate travels toward the discharge point 21 it is warmed up about 1 to 2 per convolution so that by the time it reaches the inner convolution e its temperature is about 172 C. Meanwhile, the air discharged into the inlet manifold 32 undergoes a temperature drop of 8 to 10 and a pressure drop of about 5 pounds per square inch, thus passing below the condensation point under the particular operating conditions.

The continuously increasing ebullition of the condensate in the spiral passage a-e drives 011 the more volatile nitrogen and argon constituents remaining in the condensate, along with relatively small amounts-of oxygen, and consequently the condensate is progressively enriched as it travels toward the discharge point 21. The rate of flow of the condensate is such that by the time it reaches the inner convolution e only substantially pure liquid oxygen remains, and the gaseous oxygen evolved at an adjacent to the point 21 and inner convolution e passes into the outlet pipe 34. Thus, as the condensate travels from the outer convolution a to the inner convolution e, its oxygen content increases from approximately 90% to better than 99%. Due to the accumulation of gaseous oxygen above the liquid level at and adjacent to the discharge point 21, the volatilized nitrogen and argon constituents, along with relatively small amounts of oxygen, are forced to travel through the spiral passage above the liquid level in a direction counter to the fiow of condensate and thus escape through the admission end of the channel back into the column 2 and the heat exchanger 5.

The condensed air in the manifold 38 is forced through the pipe 4 and heat exchanger 5, then through a restricted orifice or the like device in the coupling 8 where a substantial pressure drop takes place, and finally flows through line '1 into distributor 3 which sprays or otherwise distributes the liquefied air into the column 2 for the first stage rectification. The flow of fluid through the column 2 is effective, as above noted, to separate the major portion of nitrogen and argon from the oxygen so that the condensate reaching the boiler has a relatively high oxygen content, approximately 90%. The volatilized nitrogen and argon constituents, along with a small amount of oxygen, pass upwardly through the column 2 into the heat exchanger 5 and thence to other parts of the system where it is used to extract heat from the incoming compressed air, thus enhancing the over-all emciency of the system. 1

The design of the system herein shown is such as to insure a close approach to thermodynamic reversibility and hence permits the production of an oxygen product of higher purity than would be possible with a conventional system using air compressed to the same pressure.

An advantageous feature of the design herein shown is that the fractionating boiler accomplishes the work of a number of theoretical plates in a column, the number depending on the conditions of operations of the column, such as the reflux ratio. The actual number of plates replaced will be greater than the theoretical number since the former do not have 100% efiiciency, particularly when operating under conditions for attaining the maximum. possible recovery of oxygen (approximately 63%) from the incoming air. Although the number of plates replaced can only be determined empirically, the saving under any operating conditions will be appreciable so that the overall unit (boiler, column etc.) will be shorter or of smaller size than would otherwise be possible.

Since the operating principles herein disclosed may be advantageously employed for effecting the separation of components having diiferent boiling points from a condensible fluid, the apparatus herein shown, with or without modiflcation, may be used for a wide variety of applications, irrespective of whether the fluid mixture is normally a liquid or a gas. It is to be understood that the present disclosure is for the purpose of illustration and that various changes and modifications may be made suitable for the particular application, without departing from the spirit and'scope of the invention as set forth in the appended claim.

I claim:

Apparatus for separatingrthe components of a liquefied fiuid mixture, comprising an enclosed spiral channel, means at the outer end of said spiral channel for admitting liquefied fluid, means operative to provide a passage of minimum height .between the top of said channel and the level of liquid therein, said passage permitting the escape of vaporized fluid from said channel, means for withdrawing fluid from the inner end of said channel, a spiral conduit disposed within and in spaced relation to the side walls of said spiral channel, means for admitting fluid to the inner end of said conduit, means for withdrawing fluid from the outer end of said conduit, a rectifier, means for conducting fluid from the outer end of said spiral conduit to said rectifier, and means for conducting condensate from said rectifier to the outer end of said spiral channel.

HOWARD O. McMAHON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 924,428 Claude June 8, 1909 1,238,053 Smoot Aug. 21, 1917 1,492,063 Barbet Apr. 29, 1924 FOREIGN PATENTS Number Country Date 14,303 Great Britain June 27, 1903 338,964 France Aug. 1, 1903 148,302 Great Britain May 5, .1921 0t 1921 

